Conventionally, there are demands for accurately obtaining a remaining electrical charge of a secondary battery for backup or power source of various devices or a secondary battery such as a lead battery mounted on a vehicle. Generally, in the secondary battery, as there is a correlation between a remaining electrical charge and an open circuit voltage, the remaining electrical charge can be estimated by obtaining the open circuit voltage. However, the open circuit voltage of the secondary battery has to be obtained while charge or discharge is not performed, and it takes long time until the open circuit voltage becomes stable after charge or discharge is finished. Therefore, there has been proposed a way of measuring the open circuit voltage under predetermined conditions for a short time and using a function for approximately obtaining the open circuit voltage with time characteristic to obtain a convergence value of the open circuit voltage (see, for example, patent documents 1 to 3).
In order to obtain the convergence value of the open circuit voltage of the secondary battery by the above-mentioned way, its accuracy depends on the accuracy of the function used in approximate calculation. For example, a function of time characteristic, such as a polynomial function or a logarithmic function, is usually used as a basis to obtain the convergence value of the open circuit voltage. However, it is difficult to approximate the open circuit voltage of the secondary battery with time characteristic with high accuracy, and the obtained convergence value of the open circuit voltage might show a large error. Hence, when the above-mentioned way is used in estimating of the remaining electrical charge of the secondary battery, high accuracy is hard to be assured under the constraints of function used in approximate calculation and the remaining electrical charge cannot be estimated accurately, which presents a problem.
Meanwhile, the above-listed patent document 4 discloses a way of measuring a plurality of voltages along the time axis within a predetermined time period after completion of charge or discharge of a secondary battery, using the voltages in successive calculation to obtain a convergence value of the open circuit voltage of the secondary battery and using a 4th order or more exponential decay function so as to approximate a time characteristic of the open circuit voltage of the secondary battery.    [Patent document 1] Japanese Patent Laid-open Publication No. 7-98367    [Patent document 2] Japanese Patent Laid-open Publication No. 2002-234408    [Patent document 3] Japanese Patent Laid-open Publication No. 2003-75518    [Patent document 4] Internal Publication No. WO2005/006006